Just wondering if anyone has ever seen this before. While flying I had a sudden change in engine/prop sound. I quickly landed expecting to find a loose or cracked muffler but found my prop split from one tip through the hub and a small ways into the opposite blade. I have never seen that before????

I must confess that the prop has seen a lot of use and the "clear coat" has been peeling/flaking off for a while. I would occasionally give it a light sanding and seal with CA and re-balance if neccessary. I would quess this prop has had several hundred flights over the last couple of years.

Steve

I have seen this happen in laminated props when the bolts are tightened down too far. You do not see the split when you do this but you will hear it when it happens. You can fly with the split for a long time before it shows up.

As you know, this was not a laminated prop and try to torque the bolts as evenly as possible "by hand" but not too tight. I am guessing it may have just been an age factor coupled with periods of bare wood exposure where the finish had peeled off which allowed the wood to dry, age, weaken....maybe???????

Steve

I've seen it happen with wood props that had some time on them. Typically the hub had been compressed quite a bit because the owner failed to use the correct amount of torque when tightening the hub bolts. This happened over and over until the prop washer was welll sunk into the prop hub. Not saying this happened on yours but it's a general cause of wood prop failures that needs to be considered.

A good many people simply run the hub screws down until they are "tight". Usually that level of torque is 3 or 4 times the amount it should have been, continually compressing the prop hub until a small longitudinal split occurs in the hub. Later it travels through the rest of the prop.

Multi bolt hubs should be torqued to 40 inch pounds. Single bolt hubs should have an anti rotation pin installed in the hub to eliminate any slippage and then use the same torque. Props should be checked periodically and discarded when the hub has been seriously compressed or any signs of a split become apparent.

On another tack, you could have struck the prop on something at one time or another starting a tip split as well. It just took some time for the effects to show up.

Pat.....The hub on this prop was "compressed" a little from the prop washer but all my wood props do that over time. I dont think I am overtightening but I will probably look into either borrowing or buying a decent small torque wrench to get a "feel" for 40"/lbs. There may have been some "ground nicks" over the years with this prop in question as well. It was probably a combination of things that finally said enough is enough....lol. I'm just glad I noticed the "change in the air" when it happened and got it down quickly with no real damage.

Thanks,
Steve

......... and what effects do those occasional prop "rips" have over time? Can't be good for the prop either.

GOOD information.... I am purchasing a tourqe wrench...

40 inch pounds is between 3 and 4 foot pounds. About the time a hex wrench snugs up to the hub you made 40 inch pounds. The down side is that props have to be checked about every four or five flights for bolt security. Add a spinner over the prop and it's pretty clear why prop bolts don't get much attention. They end up an install and forget item, which is exactly opposite of what they should be. Full scale and uav's have checking the prop security before every flight part of the pre flight process, and the reality is our gassers are no different. Many uav's use engines the same size as what we use. Ifthey feel they need to check the propeller before every flight, perhaps we should consider doing it a bit more often that we do.

The longer a prop is used the deeper a prop washer will sink into a wood hub. That time span is extended considerably if the correct torque amount is used. Those that crank down on a new wood hub do about 4 months of hub compression in the first installation. Each successive installation of the prop compresses the hub even more. At a certain point a split starts at the hub. At 40 inch pounds there is generally only a slight marring of the finish on a wood prop, but no compression.

Many of us store our models indoors, and the process of moving them in and out has been noted as a danger period creating a lot of "hanger rash" Typically propellers, rudders, and horizontal stabs manage to run into door frames and parts of automobiles during the transport process. Each one of those dratted moments has the ability to generate minor propeller damage that won't be discovered until after the prop reaches a high rpm level and loads up. That's when that invisible stress fracture becomes very visible...

Propellers most certainly do wear out, so checking them often is a good thing.

Thanks. I learned a bunch from this topic. I was always taught that the prop bolt isn't tight enough until the prop digs uncomfortably into your hand.

Pat, that is exactly the right answer.
Having done a lot of bow building, I can add to that, once the wood has compressed slightly (taken a "set" ), it has hardened quite a bit, and will resist further compression IF the bolts are torqued to the same specification time and again. This is the very max that the wood will stand in compression.
If the wood compression will not settle down, the initial torque was too high, and eventually the prop will fail.


I have spent a lot of thought on this, and came up with the following idea:
Wood being what it is, is very well suited for a prop, but is lacking in resisting the compression force that it takes to make a proper friction load to convey the engine torque without slippage and eventually breaking either bolts, or prop. The wood certainly will compress before non-slippage is achieved. Especially the "blond" wood, which is lighter, but also softer.
To make the wood compression resistant, steel compression tubes could be epoxied into the hub mounting holes to take the screw compression loads, and transmit the torque to the prop. Now the screws can be fully loaded to 80% of their strain limit without crushing the wood. The steel insert tubes will also enhance the centrifugal force integrity of the wood prop.

pe reivers: Sounds like you got a good idea there. Seems like with the steel tubes...a person would not need to re-torque the bolts as often. Thanks , Capt,n

Good idea. Here's another item that I'm sure you've already considered. With the multi bolt hubs compression and clamping force really may not be that much of a problem. The single bolt hubs have more of a clamping force problem if they don't have an additional anti rotation pin in the hub.

I've been thinking that the dis-similar metals between the prop hubs and the prop bolts may be what's causing things to loosen up. I don't believe that a correctly torqued prop bolt is compressing the wood more as it's used, but the difference between steel and aluminum may be playing a big part. Perhaps stainless steel screws may be better suited to the task with aluminum hubs.

I didn't know you made bows, Pe. Now that's craftsmanship!!

I do not think that the compression strength perpendicular to the grain of i.e. beechwood is high enough to resist the load needed to avoid slippage in a 50cc engine or above. That is, on the small diameter prop drive disks that we use. One solution would be to increase disk diameter, but that would increase the prop billet size and hence the prop price.
I have tried hardwoods like Ipe, and they resist pressure quite well, but are too grain sensitive to make good props. They will splinter on the slightest impact and are harder to machine than beech or maple.

On the bolt thread strain distribution:
The tensile load in the bolt tends to strain the bolt, so the thread pitch will increase slightly, to the extend, that only the very first threads do any serious load carrying. Using different (harder) steel for the bolts is not the solution, because the tensile E-modulus for all steels is nearly constant. That means, that higher tensile steel bolts can be torqued higher, but also will stretch more under the increased load, causing a larger pitch change.
The solution could be in the use of helicoils (in theory), which deform slightly, and thus distribute the load better over a larger number of threads. From a practical point of view, I hate helicoils for bolt connections that have to be loosened time and again.
Another solution can be, to reduce the bolt diameter between head and threads, so the thread section is lowly loaded. Drawback of this solution is, that the now much thinner bolt stem cannot resist the mounting torque all that well.
Yet another solution is to insert steel nuts in the back of the prop drive washer. These nuts only have a height of 0.8 times the thread diameter, so bolt stretch issues become smaller. They also allow some movement when the bolt is loaded sideways, and thus largely prevent the fatigue failure of our prop bolts if prop slippage occurs.